1. Field of the Invention
This invention relates to a servo control device, and more particularly to a gain control technique for a servo control device installed in an optical information recording and reproducing device.
2. Description of the Prior Art:
Normally, in a device for recording or reproducing information on or from a disc such as a Compact Disc or a Laser Disc, servo control is very important for accurately reading out information recorded on a disc. Servo control circuit generally includes a closed servo loop for feeding back information relating to a relative speed of a pickup for reading information with respect to the disc, information relating to a position of the pickup with respect to tracks or information relating to focusing condition of objective lens. In such a servo control technique, an error signal is generated based on a read-out signal from pickup, and it is necessary to control gain of the servo loop so that error signal becomes an appropriate voltage level. For this reason, an automatic gain control circuit is frequently provided in a servo control device. Recently, some types of automatic gain control circuits have been proposed. One of automatic gain control circuits detects magnitude level of an error signal and carries out gain control so that error signal supplied to a servo equalizer circuit becomes an appropriate magnitude level. However, in use of such a circuit, gain of servo loop largely varies, due to characteristic differences of individual pickups or circuits and/or temperature characteristics, after the servo loop is closed, and therefore gain control cannot be accurately performed. In this view, an automatic gain control utilizing disturbance source is advantageous. In such gain control, disturbance signal having a given magnitude level and a given frequency is mixed into servo loop, and disturbance signal traveled through servo loop is picked up and its magnitude level is detected, with a delay time, so as to detect a loop gain peculiar to the servo loop including a disc. Then, level of error signal is varied by an attenuator or the like to be an appropriate level for a following servo equalizer.
In a known automatic gain control device, gain control described above is carried out in a certain time period prior to reproduction of disc. In an automatic gain control device utilizing disturbance source, a pickup mechanically fluctuates dependent upon the disturbance signal after mixing the same. However, disturbance signal is of audible frequency and is mixed into servo loop prior to reproduction of audio information (i.e., no music period), and hence disturbance signal may leak in audio information to be reproduced and sensed by listener as discordant and uncomfortable noise. This is illustrated in FIG. 1. As illustrated in FIG. 1, a disc is set in a reproduction device at time t.sub.1, and the focus servo is closed at time t.sub.2 so that a light beam from an pickup is focused on a track by an object lens. At time t.sub.3, tracking servo loop is closed and the pickup starts following the tracks. When the focus servo control and the tracking servo control are completed, automatic gain control commences. However, during the automatic gain control soon after the time t.sub.3, MUTE signal is effective and no audio information is output. Namely, no music is reproduced before searching a head portion of music to be reproduced, and in this period disturbance signal is audible as discordant noise.
FIG. 2 is a diagram illustrating a relation between a position of an objective lens with respect to a pickup and a level of error signal. In FIG. 2, X-axis represents a deviation, i.e., an offset, measured from a lens-center which is a center of a lens movable range on a pickup, and Y-axis represents a level of tracking error signal. Namely, FIG. 2 illustrates variation of tracking error signal corresponding to position of lens. As seen from FIG. 2, level of tracking error signal largely varies depending on an offset from lens-center, and therefore a servo control device is required to have an automatic gain control function for absorbing variation. In an automatic gain control device, gain control is performed with no dependency upon position of lens relative to pickup. In consideration of the fact that level of error signal varies dependent upon offset from lens-center, there is developed a gain control device which arithmetically operates an appropriate gain control value from variation amount. Since variation amount is peculiar to individual discs, gain control value may be decided for respective discs. A technique of this kind is disclosed in a Japanese Patent Application Laid-Open No. 4-289520.
However, an automatic gain control device described above has the following problems. Firstly, in normal reproduction, since the pickup is positioned at an appropriate position by tracking servo control, central position of pickup rarely deviates from a target track of disc and a lens actuator corrects only very slight positional deviation of lens due to eccentricity of disc or the like. Namely, lens rarely moves largely to the extent of lens movable range, and is positioned within a narrow appropriate range (e.g., the range A in FIG. 2) near the lens-center by servo control. In other words, in normal reproduction, lens moves only within a narrow range in the vicinity of lens-center. However, in a special operation such as track jump, jumping access or the like, pickup jumps to a target track and then positioned by servo control. In this case, while pickup jumps and moves to a correct tracking position according to carriage control, lens moves largely within its movable range by fine position control, and may be positioned at an area other than an appropriate area near lens-center (e.g., range B in FIG. 2). In this situation, since level of error signal is low as shown, gain of an amplifier provided in gain control device should be increased for correctly performing servo control. However, in a normal reproduction, the pickup is positioned near lens-center and therefore loop gain set at position with large offset (largely deviated from lens-center, e.g., range B) is so high that error signal of appropriate level cannot be supplied to servo system. As a result, correct servo control cannot be performed. In addition, if carriage servo is unstable, lens temporarily stays near inner or outer edge of lens movable range, that is, shifted from lens-center. Normally, gain control is performed within a necessary and sufficient gain valuable range, and hence an automatic gain control device cannot set correct gain if lens is shifted in the above manner. This deteriorates servo characteristics and may result in a drop of product quality.
Generally, if position of pickup deviates from lens-center, level of tracking error signal asymmetrically varies. Therefore, in a reproduction of disc having relatively large eccentricity, error signal level largely varies during movement of carriage because lens rarely stays near lens-center, and hence control is unstable. Further, if loop gain is controlled irrespective of lens position in a case of disc having large eccentricity, loop gain is adjusted so that randomly detected tracking error level becomes appropriate level. Therefore, gain adjustment may be erroneous. The above description relates to problems in tracking servo control, however, there exists a similar problem in focus servo control. Namely, if lens position on pickup deviates from lens-center, sensitivity of lens varies. Therefore, if gain adjustment is carried out at an arbitrary position of disc having relatively large eccentricity, variation of controlled gain become large.